The central focus of this project is the neural basis of affiliative behaviors, such as parental care, infant attachment, and fair bonding. We have investigated the role of two neurohypophyseal peptides, oxytocin and vasopressin, in the mediation of these behaviors in various mammals. Comparative studies of the distribution of brain receptors for oxytocin and vasopressin have indicated marked differences in closely related species of microtine rodents (voles). Previously, we reported that the regional distribution of oxytocin receptors differed markedly between monogamous and polygamous voles. New findings suggest even more profound species differences for the distribution of vasopressin receptors. During this year, we have explored the functional significance of these species differences in vasopressin receptor distribution. In the monogamous prairie vole, vasopressin administered intracerebroventricularly induces both a partner preference and an increase in aggression in the male, effects which are critical to the initiation of a fair bond and which develop predictably as a consequence of mating. Central injection of a vasopressin antagonist just prior to mating blocks both the partner preference and the selective aggression, suggesting that activation of vasopressin pathways during mating is necessary for pair bonding to develop as a consequence of mating. In addition, another aspect of affiliation, paternal behavior, is selectively reduced by axon-sparing lesions of the medial nucleus of the amygdala, a region rich in vasopressin cell bodies with central projections. These new data along with our previous studies of oxytocin's role in the development of partner preferences in the female prairie vole suggest that central oxytocin and vasopressin pathways have evolved in mammals to mediate diverse forms of attachment behavior.